The present invention relates to a hydraulic booster for a fork lift truck, truck and an automobile, and more particularly to a relief valve mechanism for such a hydraulic booster.
When such a hydraulic booster is used for a fork lift truck, the control hydraulic circuit as shown in FIG. 1 may be used in which an oil pump 1 is driven by an engine 2 and sucks oil contained in a reservoir tank 3 to supply pressurized hydraulic oil through a passage 4 to a flow priority valve 5. The flow priority valve 5 supplies a portion of the hydraulic oil through a passage 6 to a hydraulic booster 8, of which the pressure is regulated by a relief valve 7. The rest of the pressurized hydraulic oil is supplied through a passage 10 to a lift control portion 11a and a tilt control portion 11b of a control valve 11. The control portions 11a and 11b are operated to control movement of the fork during loading and unloading. When a driver operates the lift control portion 11a to communicate the passage 10 with a lift cylinder 12, the lift cylinder 12 is extended to lift the fork. When the tilt control portion 11b is operated to communicate the passage 10 with a tilt cylinder 13, the tilt cylinder 13 is extended to tilt the fork so that free end of the fork extends upwardly. To lower the fork or to level the fork, the control portion 11a or 11b is operated to communicate the cylinder 12 or 13 with a drain passage 14 or 15 to contract the cylinder 12 or 13 by weight of load. A filter 16 is inserted in the drain passage 15 to eliminate foreign matters.
As will be described in detail hereinafter, a hydraulic booster 8 multiplies depressing force of a brake pedal 19 and transmits the multiplied force to a brake master cylinder 20 by the hydraulic pressure produced by throttling hydraulic oil flow in the way of the passages 6 and 17 and an operation valve 23 of a power steering 18. Master cylinder oil pressure from the brake master cylinder 20 actuates wheel cylinders 21 to brake the fork lift truck. A drain passage 22 from the hydraulic booster 8 and a drain passage 24 from the power steering operation valve 23 are both communicated with the drain passage 15.
When a steering handle 25 is in the neutral position shown in FIG. 1, the operation valve 23 of the power steering 18 communicates the passage 17 and actuating chambers 26 and 27 with a drain passage 24. When the steering handle 25 is rotated, the operation valve 23 communicates the passage 17 with one of the hydraulic chambers 26 and 27 according to the rotational direction, and communicates the drain passage 24 with the other hydraulic chamber, to perform force multiplied steering operation.
As the hydraulic booster 8 utilizes hydraulic pressure produced by throttling hydraulic oil flow between the passages 6 and 17, in order to assure sufficient quantity of hydraulic oil flow to be supplied to the power steering 18 even when the hydraulic booster 8 is in operation, and to avoid damage of the brake system due to excessive hydraulic pressure, the hydraulic booster 8 includes a relief valve mechanism which by-passes the hydraulic booster 8 and communicates the passage 6 directly with the passage 17.
FIG. 2 shows one example of a conventional relief valve mechanism. A relief valve B includes a main valve E which is slidable in a blind hole D formed in a booster body C perpendicularly to a port A to which the passage 17 is connected. A spring G is engaged with, and arranged between the main valve E and a plug F which closes the open end of the blind hole D, and resiliently urges the main valve E to the lowermost rest position shown in FIG. 2.
A groove H is formed on the inner periphery of the blind hole D and maintains oil flow in the port A even when the main valve E is closed. A shoulder I is formed on lower edge of the groove H.
The main valve E has a small diameter end portion J near the bottom of the blind hole D to form a shoulder K and to define a main valve chamber L which communicates with the passage 6.
A pilot chamber M is defined between the plug F and the main valve E. A valve seat N is secured to the upper end of the main valve E.
The pilot chamber M is communicated with the passage 6 through a passage O in the booster body C and an orifice P, and also with a drain passage S through a central opening Q of the valve seat N and a passage R in the main valve E.
Outside openings of the drilled passages O and S are closed by hammering seal balls T and U. The drain passage S formed in the booster body C is communicated with the drain passage 22.
In order to open or close the opening Q of the valve seat N, there is accommodated in the main valve E a pilot valve V which is urged by means of a spring W toward its closed position and actuated in response to the pressure within the pilot chamber M.
The relief valve supplies sufficient quantity of hydraulic oil to the power steering 18 in all the operating conditions of the hydraulic booster. When the throttle of the hydraulic booster 8 substantially decreases its opening area or shut the through flow of the throttle, or when the hydraulic booster is in the inoperative condition in which the throttle does not decrease the through flow rate, since the steering load of the power steering 18 is very large, oil pressure in upstream side of the throttle, i.e. in the conduit 6, is increased to reach a predetermined value determined by the force of the spring W, so that the pilot valve V is opened through the orifice P, passage O, pilot chamber M and the opening Q of the valve seat N. Thus oil flows partly through the pilot valve V and the passage S to the drain passage 22 (FIG. 1). By this, pressure is released downstream side of the orifice P, i.e. in the pilot chamber M. As the pressure in the passage 6 is directly applied in the main valve chamber L as stated previously, the pressure difference between the main valve chamber L and the pilot chamber M causes the main valve E to move upwardly against the spring G. Thus, the shoulder K is separated from the shoulder I to communicate the main valve chamber L with the groove H. There is thus formed a by-pass passage which extends from the passage 6 to the power steering 18 through the main valve chamber L, groove H and the port A. This by-pass passage is formed besides normal passage through the throttle of the hydraulic booster 8. When the throttle of the hydraulic booster 8 substantially decreases or shut the normal passage, oil pressure in the passage 6 is increased and the main valve E forms a clearance between the shoulders K and I, and oil demand of the power steering 18 is sufficiently satisfied. Thus, the relief valve mechanism determines upper limit of oil pressure in the passage 6, i.e. on the upstream side of the throttle of the hydraulic booster 8 to protect the associated elements from damages, and to assure demand of the power steering 18 to be satisfied in all the operating conditions of the hydraulic booster 8.
In such a conventional relief valve mechanism, oil pressure on the upstream side of the throttle of the hydraulic booster 8 is introduced from the passage 6 through the orifice P to the pilot valve V which releases oil when the oil pressure exceeds a predetermined value. When the power steering 18 is operated without actuating the hydraulic booster 8, or without depressing the brake pedal 19, oil pressure on the upstream side of the throttle to be supplied to the power steering exceeds the relief value of the relief valve V. This is particularly the case when steep steering is operated. In this case, oil quantity, which is to be supplied to the power steering 18 via the normal passage through the hydraulic booster and by-pass passage through the now opened main valve chamber L, is decreased by the release of the oil through the pilot valve V. Thus, oil demand of the power steering cannot be satisfied when steep steering is operated, response characteristics of the power steering 18 is substantially deteriorated. Also, as oil quantity through the orifice P and the pilot valve V is increased, undesired noise is produced.